Solid electrolyte devices of interest usually consist of an oxygen ion conducting electrolyte membrane held between two electrodes. The examples of solid electrolyte materials include zirconia, both partially or fully stabilized by the addition of calcia, magnesia, yttria, scandia or one of a number of rare earth oxides, and thoria or ceria doped with calcia or yttria or a suitable rare earth oxide. The electrodes for these devices usually consist of porous coatings of metals such as Pt, Ag, Au, Pd, Ni and Co or metal oxides with good electronic conductivity. The electrodes participate in the charge transfer reaction between gaseous oxygen molecules or fuels (such as hydrogen, carbon monoxide or methane) and oxygen ions in the solid electrolyte by donating or accepting electrons. The electrode may also help to catalyze the reaction. For example Pt, the most commonly used electrode material on solid electrolyte oxygen sensors and oxygen pumps, shows high catalytic activity at temperatures above 600.degree.-700.degree. C. for the oxygen charge transfer reaction (O.sub.2 +4e.revreaction.20.sup.2-) at the electrode/electrolyte interface. The physical and chemical characteristics of the electrode play an important role in determining the speed of response and efficiency of the solid electrolyte devices.
The potentiometric or Nernst sensor consists of an oxygen ion conducting membrane of a solid electrolyte (with negligible electronic conductivity) such as fully or partially stabilized zirconia, and two electrodes reversible to O.sub.2 /O.sup.2- redox equilibria. If both electrodes of such a cell are exposed to different oxygen partial pressures, an emf is established across the cell which with respect to air, as the reference atmosphere is given by the nernst equation: EQU E(mV)=0.0496T Log (0.21/pO.sub.2)
where pO.sub.2 is the unknown oxygen partial pressure and T is the absolute temperature. The emf is measured by making electrical contacts to the electrodes.
Australian Patent No. 466,251 describes various geometrically distinct forms of a solid electrolyte oxygen sensor. The most commonly-used form is that of a tube, either open-ended or closed at one end, made entirely from the solid electrolyte. Other designs use the solid electrolyte as a disc or pellet, sealed in one end of a metal or ceramic supporting tube. In all cases, the reference environment, which is generally air, is maintained on one side of the tube (commonly on the inside) and the test environment is exposed to the other side of the tube.
In a potentiometric sensor, the current carrying capabilities of the electrodes are not important although electrodes with high charge transfer rates are required especially for low temperature (below 600.degree. C.) applications of the sensors. The solid electrolyte oxygen sensors with noble metal electrodes are generally used above 600.degree.-700.degree. C. Below these temperatures they suffer from slow response rates, high impedance, susceptibility to electrical noise pick-up, large errors and exceptionally high sensitivity to impurities in the flue gases.
Other solid state electrochemical devices such as oxygen pumps, fuel cells, steam electrolyzers and electrochemical reactors may consist of a tube of fully or partially stabilized zirconia electrolyte with electrodes coated both on the outside and inside of the tube. A number of such cells may be connected in series and/or parallel to achieve the desired characteristics. For example, in an oxygen pump a number of cells may operate in conjunction to increase the yield of oxygen. In a fuel cell arrangement a number of the small cells may be connected in series and parallel to increase the total current and voltage output as maximum theoretical voltage achievable from a single cell is around 1.0-1.5 volts. In all these systems, the current carrying capacity and hence the overall efficiency of the cell is determined by (i) the electrode/electrolyte interfacial resistance to charge transfer reaction and (ii) the electrolyte resistance. The interfacial resistance to charge transfer depends mainly on the electrochemical behavior, and physical and chemical nature of the electrode. Because of the high electrode and electrolyte resistance at low temperature, the cells need to be operated at temperatures in the vicinity of 900.degree.-1000.degree. C. For optimum efficiency and increased cell life it is essential that these cells be operated at lower temperatures. The voltage losses across the electrolyte can be reduced by the use of thin and mechanically strong electrolyte films. Since conventional metal or metal oxide only electrodes have high electrode resistance at lower temperatures, it is necessary, therefore, to devise better electrode materials in order to reduce overpotential losses across the electrode/electrolyte interface.